Process for the preparation of optically pure 1,2,3,4-tetrahydro-3-isoquinolinecarboxylic acid and its derivatives

A process for the preparation of an optically pure tetrahydro-3-isoquinolinecarboxylic acid derivative of formula (III) which comprises the reaction of an optically pure phenylalanine derivative of formula (I) with a formaldehyde precursor for about 10 hours to 60 hours at about 40.degree. C. to about 60.degree. C. in concentrated hydrochloric acid to give the compound of formula (II), followed by neutralization in hot water at about 70.degree. C. to 100.degree. C. with a base in accordance with the following reaction scheme:

• ##STR3## wherein in formula (I), (II), and (III):
• R.sub.1 and R.sub.2 are H, alkyl of C.sub.1 to C.sub.7, alkoxy of C.sub.1 to C.sub.7 --OCH.sub.2 O--, or halogen, which is selected from fluorine, chlorine, bromine, and iodine;
• R.sub.3 is H or alkoxy of C.sub.1 -C.sub.6.